A nitride semiconductor such as GaN, AlN, InN or a material containing mixed crystals of the nitride semiconductor has a wide band gap and is used for high output electronic devices, short-wavelength illumination devices, and the like. Among the high output electronic devices, technology related to FET (Field Effect Transistor) (particularly, HEMT, High Electron Mobility Transistor) is being developed. A HEMT using a nitride semiconductor is used in, for example, a high output/high efficiency amplifier or a high electric power switching device.
In order to attain a normally-off property, a HEMT used in such devices may include a structure having a gate recess formed by removing a part of a semiconductor layer immediately below a gate electrode. Further, there is also a HEMT including a MIS (Metal Insulator Semiconductor) structure having an insulator film formed as a gate insulator film.
For the purpose of passivation or the like, a semiconductor apparatus such as the above-described FET has a protection film made of an insulating material formed on the entire surface thereof after forming, for example, a source electrode and a drain electrode thereon.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-359256
Patent Document 2: Japanese Laid-Open Patent Publication No. 5-136126
Patent Document 3: Japanese Laid-Open Patent Publication No. 2008-306026
Normally, a silicon compound (e.g., silicon nitride (SiN), silicon oxide (SiO2)) is used as the protection film of, for example, a HEMT formed of a nitride semiconductor. The silicon compound used as the protection film has a high insulating property and a low dielectric constant and can be formed relatively easily. Further, gold (Au) is used as a gate electrode of the HEMT. The gold used as the gate electrode has high electromigration resistance and can reduce resistance of the gate electrode. Accordingly, the HEMT is configured having a protection film made of a silicon compound formed on a gate electrode made of gold.
However, a eutectic crystal of gold and silicon tends to be formed at a portion of the HEMT where gold and silicon contact each other. The forming of the eutectic crystal results in problems such as degradation of insulation property and increase of resistance at the gate electrode. Because the eutectic temperature of gold and silicon is a relatively low temperature of approximately 370° C., a eutectic crystal of gold and silicon (gold-silicon eutectic) tend to be formed during manufacture of a semiconductor apparatus or use of the semiconductor apparatus. Thereby, increase of the resistance of the gate electrode and degradation of insulation resistance tend to occur when the gold-silicon eutectic is formed. Particularly, parts of a high output electric device may locally become high temperature. Thus, the gold-silicon eutectic is easily formed in the high output electric device according to the environment or status in which the high output electric device is used. Hence, the forming of the gold-silicon eutectic results in the degradation of reliability of the semiconductor apparatus.